Polyester film base having uniform high optical density

ABSTRACT

BIAXIALLY-ORIENTED POLYESTER FILM BASE HAVING UNIFORM HIGH OPTICAL DENSITY AND FREE OF PINHOLES OR OTHER OPACITY DEFECTS IS PRODUCED BY FORMING A DISPERSION OF CARBON BLACK AND POLYVINYLPYRROLIDONE (PVP) IN ETHYLENE GLYCOL IN A HIGH SHEAR MILL, ADDING THE DISPERSION ALONG WITH POLYSTER FORMING REACTANTS TO AN ESTER EXCHANGE REACTOR, AND CARRYING OUT THE PRODUCTION OF THE POLYESTER MONOMER AND ITS SUBSEQUENT CONVERSION TO POLYESTER PREPOLYMER AND POLYESTER IN THE PRESENCE OF THE DISPERSED CARBON AND PVP.

United States Patent Othce 3,790,653

Patented Feb. 5, 1974 3,790,653 POLYESTER FILM BASE HAVING UNIFORM HIGHOPTICAL DENSITY Kenneth T. Barkey, Gerald C. Candy, and Douglas C. May,Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y. NoDrawing. Filed Dec. 29, 1971, Ser. No. 213,808 Int. Cl. B29d 7/24; C08g39/10, 51/08 U.S. Cl. 264-171 Claims ABSTRACT OF THE DISCLOSUREBiaxially-oriented polyester film base having uniform high opticaldensity and free of pinholes or other opacity defects is produced byforming a dispersion of carbon black and polyvinylpyrrolidone (PVP) inethylene glycol in a high shear mill, adding the dispersion along withpolyester forming reactants to an ester exchange reactor, and carryingout the production of the polyester monomer and its subsequentconversion to polyester prepolymer and polyester in the presence of thedispersed carbon and PVP.

FIELD OF THE INVENTION This invention relates to a process for uniformlydispersing carbon black particles in a liquid vehicle to form a stabledispersion and to manufacture of biaxiallyoriented, opaque polyesterfilm base comprising said carbon black.

DESCRIPTION OF THE PRIOR ART Many methods for producing opaque poly(ethylene terephthalate) foils, fibers, filaments, ribbons and othershaped bodies by means of dispersions of pigments, such as carbon black,and the like, are known in the prior art. US. Pat. 3,007,891, forexample, teaches finely dispersed pigment-glycol dispersions prepared byusing specific condensation products as dispersing agents in order toenhance the admixing of the dispersions into the reaction mixture usedin the manufacture of foils of polyesters containing the pigment.British Pat. 1,202,636 teaches using high shear forces in formingpigment formulations from carbon black, diglycol terephthalate and asurfactant, and subsequently using the formulations to mass color athermoplastic material, e.g. a polyester, which can be made intouniformly colored filaments or threads. Also, U.S. Pat. 3,002,942teaches a formulation consisting of carbon black dispersed in ethyleneglycol in the presence of an ionic dispersing agent which may be addedto the reaction mass of a polyester forming process before or during theester exchange or polycondensation process to manufacture a pigmentedpolyester which is useful for the manufacture of threads, fibers, andother shaped articles. Preparation of carbon black dispersions inethylene glycol by ball milling for use in preparing a polyester and afilm support of improved total diffuse density from the pigmentedpolyester is taught by U.S. Pat. 3,340,062. Also, U.S. Pat. 2,882,255teaches modified linear polyesters containing from 5 to 40% by weight ofa polymer derived in whole or in part from N-vinylpyrrolidone. Themodified resins are used to make cast films and spun fibers havingimproved receptivity to cellulose acetate type dyes and to acid wooldyes.

SUMMARY OF THE INVENTION When finely-divided carbon black is milled in adispersing mill under high shear conditions in a minimum amount ofethylene glycol vehicle, the highly concentrated dispersion has atendency to form a gel in the mill, making further uniform dispersing ofthe carbon particles difficult. It has now been found that by carryingout the high-shear milling of carbon black, preferably a furnace carbonblack, in ethylene glycol in the presence of dissolvedpolyvinylpyrrolidone, (PVP), as a dispersing agent amounting to fromabout 2 to 8 percent by weight of the carbon black-PVP-ethylene glycolcharge to the mill, a carbon black dispersion in ethylene glycol isobtained which does not gel. The dispersion has advantageous propertiesfor use as an opacifier means for the manufacture of opaque,biaxially-oriented film, particularly useful as a support in amulti-layered photographic film. The dispersion of the presentinvention, in a preferred form, consists essentially from about 15 to 20parts of carbon black, from about 4 to 5 parts of polyvinylpyrrolidone(PVP) having a molecular weight of from about 10,000 to 30,000, and fromabout 76 to 80 parts of ethylene glycol, all parts being by weight.

The dispersing of a finely-divided pigment, such as carbon black, into aliquid vehicle, such as ethylene glycol, is a mechanical operation whichin itself, to a certain degree, is readily accomplished with availablecommercial mills. Formation of stable dispersions of the dispersedcarbon black in ethylene glycol vehicle is aided, however, by the use ofdispersing agents, such as surfactants, which serve to enhance wettingof the individual carbon particles by the liquid and to prevent thedispersed particles from agglomerating after the dispersing mill isturned off and the dispersion is allowed to stand. Use ofsurfactant-containing carbon black dispersions in the manufacture ofopaque polyesters, such as poly(ethylene terephthalate), which are to beused to produce opaque fibers or filaments is generally successfulbecause of the final form of such products. However, when dispersionsare to be used to opacify sheets, foils or films which are extremelythin and which must be biaxially-oriented, the need for a highly uniformdispersion of the opacifier in the polyester becomes more critical.Inadequate dispersion of the opacifier in a polyester intended for useas a film support often causes production of waste film support becauseof non-uniformity in dispersions or agglomerations of particles whichcause formation of areas of differing opacity in the support. Such areasoften result from events which may have occurred sometime during themanufacture of the polyester in the presence of the dispersion, or inits subsequent extrusion into a film shaped body, or during itstentering or drawing into a biaxially-oriented film intended for use asan opaque photographic film support. A problem which is encounteredparticularly in manufacture of an opaque photographic film support witha surfactants-containing dispersion, is that the dispersant also may insome way interfere with the photographic chemistry of the film in whichthe opaque support is used. Therefore, when the polyester is to be usedto manufacture a film support the dispersant must be a material whichnot only does not interfere with the catalysis of the polyesterformation reactions, but also must be one which does not cause theformation of chemical products which may interfere with the substratumor other layers applied to the film support in its ultimate use in amulti-layered photographic film.

The dispersion of the present invention avoids the above problems andhas other advantages. The dispersion has for one advantage the propertyof not setting into a gel in the mill, thereby permitting greaterdispersibility of the individual particles of carbon black in theethylene glycol in larger amounts than previously obtained in absence ofthe PVP dispersing agent. A second advantage of the present dispersionis that when diluted with additional ethylene glycol, the dispersion canbe centrifuged, if necessary, in a high speed, for example, a Sharples,continuous centrifuge to remove any oversize carbon particles, whichhave not been dispersed by the high shear mill, while preserving thehomogeneity of the dispersion. When dispersed in the ester exchangereactants, the dispersion of the invention has been found not tointerfere with the poleyster formation. The opaque polyester productmade in the presence of the dispersion has been found particularlysuitable for the manufacture of film support for a multilayeredphotographic film. Opaque film can be made according to the presentinvention to have a specular optical density over the wavelength rangeof 350 to 750 nanometers which is in excess of 12, as compared to astandard optical density of less than about 0.05 for unpigmentedphotographic film.

In practicing the invention, a diluted, preferably centrifuged,dispersion is made and then is blended in a mixer with other polyesterforming reactant materials, i.e. dimethyl terephthalate, at atemperature above the melting point of the latter reactant material andis charged with catalysts to an ester exchange reactor wherein the esterexchange reaction is carried out in a known way to form monomer and thenprepolymer. The prepolymer is then cast, ground and polymerized furtherin a known way byfiuidized bed techniques to form an apoque form ofpoly(ethylene terephthalate) having an inherent viscosity of about 0.63.The latter polyester is melt filtered and extruded, biaxially-orientedby drafting and tentering, and is then heat set in sheet form of athickness desired for manufacture of the polyester sheet into an opaquefilm which then is used as an opaque support for a multilayeredphotographic film. The opaque biaxiallly-oriented film support is foundto be free of pinholes and of nonopaque areas, which would be foundpresent upon viewing against a lighted background if there were anynonhomogeneity of dispersion of the carbon black in the polyester.Pinholes of light, or light-passing, non-opaque, areas occur whenagglomerates of particles of a pigment break apart during the draftingand tentering of the pigmented film and leave tears or areas oftransparency in the thinlydrawn sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 Preparation ofdispersion-A furnace carbon blackpolyvinylpyrrolidone-ethylene glycoldispersion of the invention was prepared as follows:

A suitably-sized high shear mill (Kady) was used as the dispersing mill.Into the mill were placed at room temperature 76 pounds of ethyleneglycol. The mill was started. Next there were added slowly to the glycolin the mill 4 pounds of powdered polyvinylpyrrolidone (PV-P) having anaverage molecular weight of PVP of about 10,000 (commercially availableas PVP K-15, a product of GAF Corporation). Solution of the PVP into theglycol began immediately and was completed in about 5 minutes. Twentypounds of furnace carbon black in the form of carbon beads (commerciallyavailable as Conductex SC Carbon Beads, a product of Columbian CarbonCompany) were then added to the mill and milling was continued for about30 minutes, until dispersion of the carbon black particles into thePVP-ethylene glycol solution was substantially completed. The contentsof the mill were next diluted by adding 100 pounds of ethylene glycol tothe mill contents, forming a dispersion consisting, by weight, of 2%PVP, carbon black and 88% ethylene glycol. The dilute dispersion wasmixed until it was homogeneous. The fluid, dilute dispersion wastransferred from the high shear mill to a Sharples continuous high-speedcentrifuge and centrifuged to remove all oversize particles which hadnot been dispersed by the high shear mill. The amount of residue left inthe centrifuge bowl was found to weigh about 2.2 pounds. Thecentrifuged, fluid, dilute dispersion was then transferred to an esterexchange reactor charge preparation area for use in preparation of anopaque prepolymer of poly(ethylene terephthalate). Total weight of thedispersion was about 197 pounds, of which about 175 pounds were ethylene4 glycol, and 17.8 pounds were carbon black, and the remainder PVP.

Example 2 Preparation of opaque poly(ethylene terephthalate). The fluid,furnace black carbon-PVP-ethylene glycol dispersion from Example 1 wasused to make an opaque prepolymer of poly(ethylene terephthalate) bymixing 76.6 pounds of the dispersion (equivalent to about 68 pounds ofethylene glycol) into 110 pounds of dimethyl terephthalate containing aconventional ester exchange catalyst and heated at increasingly raisedreaction temperatures in the range from about 300 to 450 F. Thereactants were heated and reacted in a usual way until the esterexchange reaction between the ethylene glycol and dimethyl terephthalatewas substantially completed. The opaque monomeric-oligomeric product soformed was transferred to a polymerization reactor where the product waspolymerized to poly(ethylene terephthalate) of an intermediate level ofpolymerization (prepolymer) in a known way. Inherent viscosity of thepolyester so formed was 0.44. Melting point of this product was 257 C.The product was then blended with prepolymer products from four othersimilarly prepared batches. The blend was ground to a fine powder,fluidized, and further polymerized to form opaque poly(ethylenetrephthalate) having an intrinsic viscosity of 0.63. The yield ofproduct was 110 pounds. Polymerization in the melt phase by continuingthe prepolymerization reaction is also intended to be within the scopeof this invention.

Samples of the poly(ethylene terephthalate) product of 0.63 inherentviscosity were melt extruded through a 40 micron filter and found topass through the filter Without an undue pressure rise being obtainedover the pressure normally obtained with clear poly/(ethyleneterephthalate) of similar inherent viscosity, similarly melt extruded.This result'indicated that the particles of carbon in the polyester werewell dispersed therein by the PVP and were not agglomerated.

Samples of the poly(ethylene terephthalate) were melted, extruded intosheet form, drafted, tentered and then heat set in known ways. Theresulting biaxiallyoriented poly(ethylene terephthalate) sheet appeareduniformly opaque both upon visual examination and under magnification,as evidenced by photomicrographs of the sheet. No pinholes or non-opaqueareas were found in the sheet. Spectral optical density was found to bein excess of 8 over the Wavelength range 350 to 750 nanometers. Thesheet was found acceptable physically and chemically for use as anopaque support for photographic film of both the black and white andcolor multilayered film varieties.

While the invention has been described with ethylene glycol as theliquid vehicle for the dispersion made according to the disclosedprocess, it is to be understood that other glycols may advantageouslyalso be used for the purpose of the invention, i.e. the production of anopaque polyester sheet which in its opaque biaxiallyoriented, heat setform is to serve as an opaque support in a multi-layered photographicfilm. The term photographic film as used throughout the description andclaims is intended to include a multi-layered structure of ivhich theproduct of the invention forms at least one ayer.

Examples of other glycols which are used in the manufacture ofpolyesters and which may be used to practice the invention include thefollowing: cycle-aliphatic glycols, such as, 1,4 cyclohexanedimethanol;aromatic glycols, such as, Bisphenol A; homologous series of aliphaticglycols from C to C and, preferably, 1,2-propylene glycol,1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol; and branchedaliphatic glycols, such as, 2,2-dimethylpropanediol-1,3 and2-ethyl-2-butyl-propanediol-l,3.

The above glycols can be condensed not only with dimethyl terephthalateto form highly polymeric linear polyesters suitable for practice of thisinvention but they can also be condensed with other organicbi-functional acids, esters or acid chlorides of the aromatic oraliphatic series to form suitable polyesters.

However, it is essential that the polyester forming reactants into whichthe dispersion of this invention is dispersed be chemically compatiblewith the polyvinylpyrrolidone component of the dispersion throughout theproduction of the opaque polyester, the biaxially-oriented sheet, filmor foil made therefrom, and in subsequent use of the latter product in aphotographic film.

Examples of polyesters which have been found to be compatible in theirmelted and film forms with polyvinylpyrrolidone may be found in US. Pat.2,882,255, incorporated herein by reference, where also may be foundexamples of dicarboxylic compounds and dihydroxy compounds used in theirpreparation. It is expected that some or all of the polyesters describedin U.S. 2,882,255 will be found compatible with polyvinylpyrrolidone forthe manufacture of an opaque form of a polyester of the patent inpractice of the present invention therewith.

Any carbon black material may be used successfully in the practice ofthis invention so long as its ultimate average particle size (diameter)is Within the range of from about 5 to about 1,000 millimicrons, andpreferably within the range of from about 8 to about 80 millimicrons. Byultimate particle size is meant the size of the individual carbonparticles, and not the size of the carbon black agglomerates or beads"(the form in which the carbon black is usually provided commercially).The ultimate particle size can be determined by suspending the carbonblack sample in an organic solvent, passing the suspension through anefficient conventional pigment dispersion mill, and then quicklymeasuring the average particle size of the carbon black. Carbon black ofthe type known in the art as furnace black has been found most effectivefor practice of the invention, based on a series of tests carried outwith at least different carbon blacks obtained from various commercialsources of supply. Accordingly, furnace blacks having physicalproperties similar to those of Conductex SC are especially preferred forpractice of the present invention.

The polyvinylpyrrolidone used in practice of the invention preferably isa homopolymer which is soluble in the ethylene glycol, or other glycols,used in practice of the invention. As used in practice of the presentinvention the polyvinylpyrrolidone should not react adversely with thereactants or catalysts in the ester exchange reactor or interfere withthe rate of the reaction or cause side reactions which would interferewith the quality of the polyester product at either the monomeric,oligomeric, prepolymer, or final polyester stage, or during the melting,extrusion, calendering, drafting, tentering or heat setting of thebiaxially-oriented sheet of the invention. Furthermore, thepolyvinylpyrrolidone must not have any adverse effect on thephotographic properties of the opaque polyester sheet made by practiceof the invention when the sheet is used as a support layer in amulti-layered photographic film.

Polyvinylpyrrolidones having molecular weights in the range of fromabout 1,000 to 100,000 may advantageously be used in practice of theinvention. A polyvinylpyrrolidone having a molecular weight in the rangeof from 10,000 to 30,000 is preferred for this purpose. Thepolyvinylpyrrolidone may be introduced into the ethylene glycol inpowdered form or dissolved in a volatile solvent which will readilyevaporate from the high shear mill, or in other liquid form. In mostcases, a concentrated solution of the PVP polymer in ethylene glycol maybe advantageously used.

The amount of polyvinyl pyrrolidone dispersing agent used in making theopacifier dispersion of the present invention will normally be only thatamount needed to be added to the carbon black-glycol mixture to ensureadequate dispersion of the carbon black particles in the glycol underhigh shear milling conditions to form a stable, nongelling fluiddispersion. Usually one part by weight of polyvinylpyrrolidone per about3 to 6 parts of carbon black particles by weight will be adequate. Morethan the above ratio of polyvinylpyrrolidone dispersing agent to carbonblack may be used, but such use is not necessary to achieve theadvantages of the invention. In practicing the invention, thepolyvinylpyrrolidone is preferably first dissolved in the entire amountof glycol to be used to make the dispersion in the high shear mill andthe carbon black is then blended in.

The amount of carbon black particles dispersed into the glycol vehiclein the high shear mill preferably is that amount which can be dispersedhomogeneously in the glycol by the polyvinyl pyrrolidone dispersingagent without causing dry agglomeration of the particles. The maximumamount can be determined experimentally for a particular glycol vehicle.In the case of ethylene glycol, the amount of carbon black particleswill usually be one part by Weight of carbon black per about 3 to 6parts by weight of ethylene glycol.

When the milled carbon black dispersion is used as the opacifierdispersion in the ester exchange reactor, the dispersion will be used inan amount which will be adequate to cause opacification of the esterexchange product and the polyester thereafter made at each stage of themanufacture and in the ultimate product, i.e. the biaxiallyoriented filmused as a support in a multi-layered photographic film. Usually about 5pounds of carbon black dispersed with 1 pound of polyvinylpyrrolidonedispersing agent will be charged to the ester exchange reactor withabout 20 pounds of ethylene glycol for each pounds of opaque prepolymerformed, in order that the ultimately prepared polyester film supportwill have a specular optical density over the wavelength range of 350 to750 nanometers in excess of 8.

Multi-layered photographic film of the type in which the opaquebiaxially-oriented polyester film made in accordance with the presentinvention serves as a support, or base, is well known in thephotographic art. Such film is taught, for example, in U.S. Pat.3,340,062, FIG. 9.

While the opaque polyester made according to the present invention isparticularly advantageous in its use as a support in the manufacture ofmulti-layered photographic film, it is to be understood that the opaquepolyester hereof also may be used in the manufacture of other opaqueshaped 'bodies, such as filaments, fibers, foils, and ribbons and othershaped bodies in which the high optical spectral density of thepolyester may be found to be advantageous.

The invention has been described in detail with particular references tothe preferred embodiments thereof but it will be understood thatvariations and modifications can be efiected within the spirit and scopeof the invention as described hereina'bove and as defined in the claims.

We claim:

1. In a process for making opaque, biaxially-oriented, heat-setpolyester film suitable as a support material in a multi-layeredphotographic film, said process comprising the steps of: 1) making adispersion of carbon black particles in a glycol vehicle in a high shearmill; (2) mixing said dispersion with polyester forming reactants andcatalysts to form a mixture for forming a polyester monomer by an esterexchange reaction; (3) charging said mixture to an ester exchangereactor and heating and reacting said mixture to form said monomer; (4)condensing said monomer and polymerizing the condensed product to anopaque polyester having an inherent viscosity of about 0.63; (5) meltingand extruding said polyester into an opaque sheet form; and (6) drawing,tentering, and

heat setting said sheet into said opaque biaxially-oriented, heat-setfilm, the improvement which comprises carrying out said dispersionmaking step in the added presence of a carbon black dispersing amount ofpolyvinylpyrrolidone as a dispersing agent, said polyvinylpyrrolidonehaving a molecular weight in the range from about 1000 to 100,000 and'being present in the ratio of at least one part by weight ofpolyvinylpyrrolidonc per about 6 parts by weight of carbon black.

2. The process according to claim 1 wherein saiddispersion in said millcomprises by weight from about 15 to 20 parts of carbon black, fromabout 4 to 5 parts of polyvinylpyrrolidone, and from about 76 to 80parts of glycol.

3. The process according to claim 1 wherein the reactants for formingthe polyester monomer comprise ethylene glycol and dimethylterephthalate.

4. The process according to claim 2 wherein the glycol is ethyleneglycol.

5. The process according to claim 4 comprising diluting the dispersionwith ethylene glycol and passing the dilute dispersion through acentrifuge to remove oversized particles.

References Cited UNITED STATES PATENTS 2,882,255 4/1959 Caldwell et a1260-873 3,640,944 2/1972 Seppala et a1 260-40 R 3,002,942 10/1961Zoetbrood 26022 A 3,492,253 1/1970 Katz et al 260-40 R 3,340,062 9/1967Hunter, Jr., et a1. 96--84 M 2,882,253 4/1959 Leiferdink et al. 264783,377,412 4/1968 Franks 264-'/' 8 ROBERT F. WHITE, Primary Examiner J.'R. THURLOW, Assistant Examiner US. Cl. X.R.

260-334, 40 R, T; 264-210 R, 211, 289, 349

